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NortWind t1_j1x4th5 wrote

The reaction from limestone with water is endothermic, it consumes energy. Limestone is made by tiny fossil shells piling up on the ocean bottom.

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capt_yellowbeard t1_j1x8oo7 wrote

Actually this isn’t quite correct. The limestone must first be burned to make calcium oxide (that is energy in) and then when it’s mixed it creates an exothermic reaction when hardening.

Limestone is itself already chemically similar to hardened concrete.

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NortWind t1_j1x9c1l wrote

Right. But that is not limestone reacting with water, it is CaO (aka quicklime) reacting with water.

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